1. Field of the Invention
This invention relates to the piezoelectric crystal elements of shear mode and the process for the preparation thereof. The single crystals can be PMN-PT (Lead Magnesium Niobate-Lead Titanate), PZMN-PT (doped PMN-PT), or related piezoceramic materials. More particularly, the present invention relates to the discoveries of the new cut directions that optimize the shear mode piezoelectric properties. In the discovered cut directions, the PMN-PT crystal elements and related compositions have super-high piezoelectric performance with d15, d24 and d36 share mode at room temperature. Even more particularly, the present invention relates to a d15 shear mode crystal that gives the maximum d value and is free from the cross-talk of d11 and d16. A further aspect of the present invention is that the d36 mode provides substantial reliability over other shear elements due to its re-poling capability. The crystal elements above can be commercially used for high-sensitive acoustic transducers and in many other applications known to those of skill in the piezoelectric ceramic and ceramic composition arts.
2. Description of the Related Art
The piezoelectric materials are the operational center of acoustic transducers which are broadly used in medical and commercial imaging systems and SONAR systems. The most common types of transducers utilize lead zirconate titanate (PZT) based ceramics as a piezoelectric function. Piezoelectric ceramics convert mechanical energy into electrical energy and conversely electrical energy into mechanical energy. While conventional PZT materials remain the most common materials used in acoustic transduction devices, changing material requirements have fostered the need for new piezoelectric materials having improved dielectric, piezoelectric and mechanical properties.
In THE early 1980s, Kuwata et al. (see J. Kumata, K. Uchino and S. Nomura, Dielectric and piezoelectric properties of 0.91Pb(Zn1/3Nb2/3)O3-0.09PbTiO3, Jpn. J. Appl. Phys., 21, 1298-1302 (1982)) found relatively “high” piezoelectric coefficient, d33, of 1500 pC/N and electromechanical coupling factor, k33, of 0.92 in 0.91PZN-0.09PT single crystals along <001> direction. The entire disclosure of Kuwata is incorporated herein by reference.
Later, relatively “high” piezoelectric properties were also observed in PMN-PT crystals by Shrout and his co-workers in 1990 (see T. R. Shrout, Z. P. Chang, N. Kim and S. Markgraf, Dielectric behavior of single crystals near the (1-x) Pb(Mg1/3Nb2/3)O3-xPbTiO3 Morphotropic Phase Boundary, Ferroelectrics Lett., 12, 63-69 (1990)), but substantial limitations remained during application and testing The entire disclosure of Sprout et al., is herein incorporated hereby reference.
Reasonably “high” electromechanical coupling (k33)>90%, piezoelectric coefficient (d33)>2500 pC/N and increased strain up to 1.7% in <001> orientation (poling along <001> axis) were reproducibly observed in the later 1990's (see S. E. Park and T. R. Shrout, Ultrahigh strain and piezoelectric behavior in relaxor based ferroelectric single crystals, J. Appl. Phys., 82, 1804-1811 (1997)). The improved “high” piezoelectric properties noted in this literature promised a new application of acoustic transduction devices using the longitudinal extension mode (d33 or compression mode) but failed to achieve the present results. The entire disclosure of S. D. Park is herein incorporated by reference.
The shear mode of piezoelectric vibration is broadly used in acoustic actuators and sensors. For examples, accelerometers utilizing the shear principle have some special advantages compared to the standard compression type accelerometers as they are considerably less strain sensitive to mounting conditions. Unfortunately, the shear piezoelectric coefficient d15 for <001> oriented PMN-PT crystals is very small, less than 200 pC/N (see Rui Zhang et al., Elastic, piezoelectric and dielectric properties of multi-domain 0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3 single crystals, J. Appl. Phys. Vol. 90 (2001) 3471-3475). The entire disclosure of Zhang is herein incorporated by reference.
However, the super-high shear piezoelectric coefficient d15 for <111> oriented PMN-PT crystals was discovered as high as 8000 pC/N for PMN-33% PT crystal (Pengdi Han, Progress in PMN-PT crystal growth, 2002 U.S. Navy workshop on acoustic transduction materials and devices, 13˜15 May, 2002 Penn State.) The entire disclosure of Han is herein incorporated fully by reference. While this piezoelectric coefficient d15 is one order higher than that of traditional PZT piezoelectric ceramics (the maximum d15 of PZT-5H is typically 750 pC/N), this improvement limited in understanding and nature, as will be discussed hereinbelow.
Soon after, it was confirmed that the d15 could be as high as 4100 pC/N for PMN-30% PT crystals (see Rui Zhang et al., Single domain properties of 0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3 single crystals under electric field bias, Appl. Phys. Letters Vol. 82 No. 5, February (2003)). The entire contents of Zhang et al., are herein incorporated fully by reference. As with Han above, Zhang fails to provide a full understanding of the increased d15 measure.
Recently, the d15 was also observed as high as 5980 pC/N for PMN-31% PT crystals (see Jue Peng et al., Shear mode piezoelectric properties of 0.69Pb(Mg1/3Nb2/3)O3-0.31PbTiO3 single crystals, Solid State Communications 130 (2004) 53-57). The entire contents of Peng et al. are herein incorporated by reference. Peng et al. fails to provide the necessary understanding and additional elements to prevent cross talk and improve reliability.
US 2005/0034519 A1, Feb. 17, 2005 to Ken Kan Deng et al., the entire contents of which are herein incorporated by reference) discloses an acoustic vector sensor, specially an underwater acoustic vector sensor using a shear mode (d15) PMN-PT crystal. However, as with each of the disclosures noted above, there is no information of crystal orientation and cut direction details.
In view of the related references, it is clear to those of skill in the art that none provides a report of preparation and application for a d36 shear mode of piezoelectric crystals.
As is also clear from the references themselves, all of the d15's tested or calculated above are based on the common orientation: <111> as poling direction (3 axis) and <110> as applied field direction (1 axis). These references also illustrate the severe lack of investigation to determine an optimum direction (orientations) which give the optimized piezoelectric performance for each piezoelectric vibration modes.
As a consequence, there is a need to both optimize multiple piezoelectric performance indicia and calculate an optimum direction. In response to these needs, in this invention, we report the discovery results of the new cut directions that maximize piezoelectric coefficients, including a d36 mode, for all of the possible symmetric domain configurations of PMN-PT related crystals.